Plasma arc torches are used in many commercial operations to generate very high temperature streams of gas, to spray molten metal onto machine parts, to cut materials in the same manner as a cutting torch, and to provide reliable ignition for large coal-fired boilers.
Some applications of plasma arc torches are based on high temperature reaction between high temperature gas and solid particles fed through the torch. Some mixtures of two or more solid material can be fed to the torch and formed into a new compound under the high temperature conditions produced by the plasma arc.
In a plasma-arc operation, a stream of gas is ionized, thereby forming a conductive plasma which may assist in transporting fine solid particles.
Very high temperatures are produced in the plasma-arc, and may be as high as 3850.degree. C. (7000.degree. F.) with the temperature of solid particles leaving the torch being as high as 2750.degree. C. (5000.degree. F.), well above the melting temperature of most solid materials.
One potential product of plasma arc processing is strontium hexaferrite (SrO.6Fe.sub.2 O.sub.3), which is used for making ceramic-type ferrite magnets. If about 6 moles of high purity (about 99.6% pure) hematite (Fe.sub.2 O.sub.3) is mixed with one mole of high purity (about 98% purity) strontium carbonate (SrCO.sub.3) and the mixture is passed through a plasma torch, then most of the mixture of melted particles, on leaving the torch and solidifying, is converted to strontium ferrite, usually to the extent of about 30 to about 70% of the feed.
A much more interesting feed consists of a mixture of 6 moles of hematite, 1 mole of celestite (strontium sulphate), and 1 mole of sodium carbonate or calcium carbonate. When this mixture is smelted conversion to strontium hexaferrite is usually between about 70% and 90%. The sulphate radical from celestite is combined with either the sodium or calcium carbonate to produce sodium or calcium sulphate.
The strontium ferrite which results from these procedures is ground so that impurities can be magnetically separated from the product, since the unconverted hematite, residual strontium oxide and other impurities, such as sodium or calcium sulphate, are non-magnetic while the strontium ferrite is highly magnetic.
If desired, the heat contained in the product, which may leave the arc at a temperature of about 2750.degree. C., can be partially recovered from cooling water and recycled hot gases.
One of the problems of such plasma arc processes is the extreme temperature of the product emanating from the plasma gas stream and the inefficient recovery of heat in the plasma gas stream. In addition, if impure starting materials are used, the impurities become associated with the product and are difficult to separate adequately. In the case of strontium ferrite, another problem is the extremely aggressive nature of the molten material, which is a good solvent for almost all refractory materials, such as aluminum oxide, magnesium oxide and silica.